MAINTAINERS: Update maintainer entry for keystone platforms

Switch the kernel tree for keystone to the consolidated ti tree and add
myself as primary maintainer for keystone platforms to offset Santosh's
workload.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Link: https://lore.kernel.org/r/20211123001725.21422-1-nm@ti.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>